This invention relates to piezoelectric surface electroacoustic surface wave devices and systems which make use of such devices and more particularly to electroacoustic surface wave devices comprising arrays of interdigital electrode structures deposited either between or upon the surface of piezoelectric and non-piezoelectric substrate structures, to minimize or eliminate the composite temperature coefficient of delay of the electroacoustic surface wave device, to improve the coupling efficiency and to reduce the dispersion in devices made with layers of zinc oxide, bismuth, germanium, lithium niobate, barium sodium niobate and other well-known piezoelectric materials which will transmit sonic waves at frequencies of several hundred megacycles.
Acoustic surface wave devices offer several advantages in the construction of delay lines and filters in the UHF range in such systems as radar using linear chirp waveforms, comb structures and broad band delay lines and in systems requiring frequency response to phase coded signals, linear FM signals, nonlinear FM signals, and signals with special coding for use with matched filter devices. In these and other devices the frequency response is determined by the interdigital finger spacing and overlap of the interdigital comb structures used as input and output transducers.
It is known that an acoustic surface wave delay line can be constructed by bringing into close proximity with one another a flat piezoelectric material and a flat non-piezoelectric material having interdigital electrode structures at their mating surfaces. In the Journal of Applied Physics, volume 39, page 5400, 1968, in an article by P. O. Lopen, such a structure is disclosed; however, the temperature coefficient of delay is such that there are significant losses, hence the device is not suitable for use in applications where an extremely small or zero temperature coefficient may be achieved as in accordance with the present invention. This is important in delay line applications in which matched filters are required for pulse or phase coded operations or in generalized filter applications such as radar requiring pulse expansion and compression, communication systems requiring encoders and decoders, and band-pass and wave shaping applications requiring precise filters. The use of high coupling piezoelectric materials is desirable because it allows large bandwidth with minimum insertion loss.